Flow drill screwing is a one-sided joining technique that can be utilized to secure lightweight materials and dissimilar materials together. Many other mechanical fastening technologies, such as self-piercing rivets or clinching, are two sided joining methods, and hence require greater access to the components for joining. Regardless of the joining method, it is desirable to have mating parts flush and avoid or minimize gaps between the parts to insure a robust and high quality joint.
Flow drill screwing joins parts together by inserting the screw directly onto the surface of the assembly. The screw is rotated at a high speed of, for example, 3000-5000 rpm and a force in the range of to two to four kilo-newtons to cause localized heating of the components for joining. After sufficient heating, the screw penetrates the surface and an extruded boss is formed on the opposite side of the joint. The tip of the screw is then rotated at a lower speed of, for example, 500 to 2000 rpm to create a thread rolled surface. The speed of rotation is then further reduced to drive the screw to the seated position.
While the flow drill screwing process may be performed with or without a clearance hole, when parts are joined without a clearance hole, material tends to flow into the area between the parts. Material that flows into the area between the parts can cause separation of mating parts from each other. Hence, a gap may be created between the parts as a result of the material that flows between the parts that are joined together. Such gaps between the joined parts are likely objectionable.
This disclosure is directed to the above problems and other problems encountered when joining parts together in a flow drill screwing process particularly when no clearance hole exists prior to performing the flow drill screwing process.